<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <script>
        function fn(){
            var a = 1;
            function fnInner(){
                a++;
                console.log(a);
            }
            return fnInner;
        }

        fn()();
        fn()();
        fn()();
        fn()();

        let res = fn();
        res();
        res();
        res();
        res();

        // 我们希望在函数外部，取使用函数内部的变量。
    </script>
</body>
</html>